1. Field of the Invention
This invention relates generally to bearing members such as piston rings and more particularly to a coating for the wear surface of the bearing members to produce a wear resistant, oxidation resistant, and scuff resistant coating thereon.
2. Description of the Prior Art
High temperatures in today's internal combustion engine adversely affect the wear characteristics of the piston ring and cylinder bore. The higher the engine's R.P.M., the greater the frictional heat that is created between the piston ring and the cylinder bore. This increase in frictional heat is in addition to the normal increase in temperature due to greater compression in the engine. Because the piston ring reciprocates at such high velocity against the cylinder bore, and at such high operating temperatures, the rubbing surfaces of the piston ring and the cylinder bore tend to wear rapidly. Accordingly, great advances have been made in applying metallic coatings to the wear surface of the piston ring, such coatings being designed to decrease the friction between the piston ring and the cylinder bore.
Many flame sprayable compositions have been developed for coating the wear surfaces of piston rings. For example, one widely used composition includes a high percentage of molybdenum alloyed with various other binding elements such as shown in Prasse et al U.S. Pat. No. 3,690,686. Such coatings have improved the wear resistance of piston rings that operate at relatively low and intermediate temperatures, but it has been found that molybdenum oxides form at temperatures above 450.degree.F which are hard, abrasive, brittle and tend to dissociate from the coating. These oxides have an adverse effect on the soft cylinder bore and often causes damage by abrading the soft surface of the cylinder bore which reduces the life of the piston ring and cylinder bore.
Molybdenum has a body-centered crystalline structure which is essentially cubical in shape which is rather abrasive when used as a rubbing wear surface. On the other hand, cobalt has a hexagonal close-packed crystalline structure which is less abrasive than molybdenum. As long as cobalt maintains its hexagonal crystalline structure, abrasive oxides are not formed. However, cobalt exists in both hexagonal and cubic crystalline forms. Cobalt maintains its hexagonal crystalline form up to temperatures of about 752.degree.F and then transforms into a cubic crystalline form similar to molybdenum.
It has also been learned that the temperature at which crystalline transformation of cobalt from hexagonal to body centered cubic occurs can be altered by selectively alloying it with molybdenum. For example, the addition of 25% molybdenum to cobalt can increase the transformation temperature appreciably. Thus, a coating containing cobalt alloyed with molybdenum would be extremely desirable in engines that reach temperatures above 752.degree.F.
However, it has been found that by adding just molybdenum and other metals the desired results of low wear is not obtained automatically. Usually, intermettalic brazing and binding alloy particles are used as the starting material which serves as a bonding agent in the final coating and also forms a fairly good wear resistant coating in itself. To these alloyed particles are added specific amounts of other materials in an attempt to obtain beneficial characteristics of the added materials. Thus, experimentation must be conducted in order to obtain a coating that will give the maximum wear resistance and maximum life to the piston ring. The mere addition of cobalt and molybdenum particles to an intermetallic brazing and binding alloy and then plasma spraying the resulting mixture onto the bearing surface of the piston ring will not necessarily create the desired results. It has been found through experimentation and research that the molybdenum, cobalt, and the intermetallic brazing and binding alloy must be formed as specific particles and in specific amounts in order to form a good wear resistant coating. Cobalt is a relatively expensive metal as compared to molybdenum but, since its wear properties are so much better than molybdenum, it is desirable to have a coating that contains cobalt but which will be competitive in the industrial market.
We have discovered through experimentation that the wear resistant properties of a coating can be greatly improved by alloying molybdenum with cobalt to form one particle; alloying an intermetallic brazing and binding compound to form another particle; and then alloying the two particles with a particle of pure molybdenum. The percentage of molybdenum used in the final coating is extremely important to prevent formation of molybdenum oxides above 450.degree.F which results in an extremely abrasive coating. Therefore, the percentages of molybdenum and cobalt used in the final coating must be balanced in order to achieve high wear resistance and still produce a piston ring that will be competitive on the market.
We have also discovered that alloying of the material in the final coating can best be achieved by properly preparing the mixture to be sprayed. However, the particles in the final coating should be alloyed only with the surfaces of the adjacent particles thereby leaving distinct particles of specific materials intermixed on the final coating. This will result in a highly wear resistant coating.